Two prefabricated or artificial military harbours were taken across the English Channel from Britain with the invading army in sections and assembled off the coast of Normandy as part of the D-Day invasion of France in 1944.
The Dieppe Raid of 1942 had shown that the Allies could not rely on being able to penetrate the Atlantic Wall to capture a port on the north French coast. Thus, the Mulberries were created to provide the port facilities necessary to offload the thousands of men and vehicles, and tons of supplies necessary to sustain Operation Overlord and the Battle of Normandy. The harbours were made up of all the elements one would expect of any harbour: breakwater, piers, roadways etc.
The actual proposer of the idea of the Mulberry Harbour is disputed, but among those who are known to have proposed something along these lines is Hugh Iorys Hughes, a Welsh civil engineer who submitted initial plans on the idea to the War Office, Professor J. D. Bernal, and Vice-Admiral John Hughes-Hallett.
At a meeting following the Dieppe Raid, Hughes-Hallett declared that if a port could not be captured, then one should be taken across the Channel. This was met with derision at the time, but in a subsequent meeting with Churchill, the Prime Minister declared he had surmised a similar scenario using some Danish Islands and sinking old ships for a bridgehead for an invasion in World War I. The concept of Mulberry Harbours began to take shape when Hughes-Hallett moved to be Naval Chief of Staff to the Overlord planners.
A trial of the three eventual competing designs was set up, with tests of deployment including floating the elements, in Garlieston, Wigtownshire. The designs were by Hugh Iorys Hughes who developed his "Hippo" piers and "Crocodile" bridge units on the Conwy Morfa, using 1000 men to build the trial version; the Hamilton "Swiss Roll" which consisted of a floating roadway; and a system of flexible bridging units supported on floating pontoons designed by Major Allan Beckett RE. The tests revealed various problems (the "Swiss Roll" would only take a maximum of a 7 ton truck in the Atlantic swell). However the final choice of design was determined by a storm during which the "Swiss Roll" was washed away and the "Hippos" were undermined; Beckett's floating roadway (subsequently codenamed Whale) survived undamaged. Beckett's design was adopted and manufactured under the management of J. D. Bernal and Brigadier Bruce White, under the orders of Winston Churchill.
The proposed harbours called for many huge caissons of various sorts to build breakwaters and piers and connecting structures to provide the roadways. The caissons were built at a number of locations, mainly existing ship building facilities or large beaches like Conwy Morfa around the British coast. The works were let out to commercial construction firms including Balfour Beatty, Costain, Nuttall, Henry Boot, Sir Robert McAlpine and Peter Lind & Company, who all still operate today, and Cubitts, Holloway Brothers, Mowlem and Taylor Woodrow, who all have since been absorbed into other businesses that are still operating . On completion they were towed across the English Channel by tugs to the Normandy coast at only 4.3 Knots (8 km/h or 5 mph).
By 9 June, just 3 days after D-Day, two harbours codenamed Mulberry "A" and "B" were constructed at Omaha Beach and Arromanches, respectively. However, a large storm on 19 June destroyed the American harbour at Omaha, leaving only the British harbour which came to be known as Port Winston at Arromanches. While the harbour at Omaha was destroyed sooner than expected (due to it not being securely anchored to the sea bed), Port Winston saw heavy use for 8 months—despite being designed to last only 3 months. In the 10 months after D-Day, it was used to land over 2.5 million men, 500,000 vehicles, and 4 million tonnes of supplies providing much needed reinforcements in France.
A complete Mulberry harbour was constructed out of 600,000 tons of concrete between 33 jetties, and had 10 miles (15 km) of floating roadways to land men and vehicles on the beach. Port Winston is commonly upheld as one of the best examples of military engineering. Its remains are still visible today from the beaches at Arromanches, and a section of it remains embedded in the sand in the Thames Estuary, accessible at low tide, about 1000 m off the coast of the military base at Shoeburyness. A Phoenix unit known as The Far Mulberry sank off Pagham and lying at about 10 metres is an easily accessible scuba diving site.
 Harbour elements and code names
Below are listed brief details of the major elements of the harbours together with their associated military code names.
 Corn cob
"Corn cobs" were block ships that crossed the channel either under their own steam or that were towed and then scuttled to create sheltered water at the five landing beaches of "Sword", "Juno", "Gold", "Omaha", and "Utah". Once in position the "Corn Cobs" created "Gooseberries". The ships used for each beach were:
- Utah Beach (Gooseberry 1): Benjamin Contee, David O. Saylor, George S. Wasson, Matt W. Ransom, West Cheswald, West Honaker, West Nohno, Willis A. Slater, Victory Sword and Vitruvius.
- Omaha Beach (Gooseberry 2): Artemas Ward, Audacious, Baialoide, HMS Centurion, Courageous, Flight-Command, Galveston, George W. Childs, James W. Marshall, James Iredell, Olambala, Potter, West Grama and Wilscox.
- Gold Beach (Gooseberry 3): Alynbank, Alghios Spyridon, Elswick Park, Flowergate, Giorgios P., Ingman, Innerton, Lynghaug, Modlin, Njegos, Parkhaven, Parklaan, Saltersgate, Sirehei, Vinlake and Winha.
- Juno Beach (Gooseberry 4): Belgique, Bendoran, Empire Bunting, Empire Flamingo, Empire Moorhen, Empire Waterhen, Formigny, Manchester Spinner, Mariposa, Panos and Vera Radcliffe.
- Sword Beach (Gooseberry 5): Becheville, Courbet, Dover Hill, HMS Durban, Empire Defiance, Empire Tamar, Empire Tana, Forbin and HNLMS Sumatra.
The sheltered waters created by the Corn Cob block ships. Two of the "Gooseberries" grew into "Mulberries", the artificial harbours.
Mulberry was the code name for the artificial harbours. These were the "Gooseberries" which metamorphosed into fully fledged harbours. There were two harbours, Mulberry "A" and Mulberry "B". The "Mulberry" harbours consisted of a floating outer breakwater called "Bombardons", a static breakwater consisting of "Gooseberries" and reinforced concrete caissons called "Phoenix", floating piers code named "Whale" and the pier heads code named "Spuds". These harbours were both of a similar size to Dover harbour.
 Mulberry "A"
The Mulberry harbour assembled on Omaha beach at Saint-Laurent-sur-Mer was for use by the American invasion forces. Mulberry "A" was not securely anchored to the sea bed, resulting in such severe damage during the Channel storms of late June 1944 that it was considered to be irreparable and its further assembly ceased. It was commanded by Augustus Dayton Clark.
 Mulberry "B"
 Golden Arrow
Large floating breakwaters fabricated in steel that were anchored outside the main breakwaters that consisted of Gooseberries (block ships) and "Phoenix" (concrete caissons). During the bad storms at the end of June 1944 these broke loose, and possibly caused more damage to the harbours than the storm itself.
Reinforced concrete caissons constructed by civil engineering contractors around the coast of Britain, collected and sunk at Dungeness, the Cant and Pagham. The engineers were unable to refloat the Phoenixes and US Navy Captain (later Rear Admiral) Edward Ellsberg, already well-known for quickly refloating scuttled ships at Massawa and Oran, was brought in to accomplish the task, though not without obtaining Churchill's intervention in taking the task away from the Royal Engineers and giving it to the Royal Navy. The Phoenixes, once refloated, were towed across the channel to form the "Mulberry" harbour breakwaters together with the "Gooseberry" block ships. Ellsberg rode one of the concrete caissons to Normandy; once there he helped unsnarl wrecked landing craft and vehicles on the beach.
The dock piers were code named "Whale". These piers were the floating roadways that connected the "Spud" pier heads to the land. The roadways were made from torsionally flexible bridging units that had a span of 80 ft., mounted on pontoon units of either steel or concrete called "Beetles". After the war many of the "Whale" bridge spans from Arromanches were used to repair bombed bridges in France, Belgium and Holland. Such units are still visible as a bridge over the Meuse River in Vacherauville (Meuse), as a bridge over the Moselle River on road D56 between Cattenom and Kœnigsmacker (Moselle) and in Vierville-sur-Mer (Calvados) along road D517.
Beetles were pontoons that supported the "Whale" piers. They were moored in position using wires attached to "Kite" anchors which were also designed by Allan Beckett. These anchors had such high holding power that very few could be recovered at the end of the War; the only known surviving one is displayed in a private museum at Vierville-sur-Mer.
 Spud Piers
The pier heads or landing wharves at which ships were unloaded. Each of these consisted of a pontoon with four legs that rested on the sea bed to anchor the pontoon, yet allowed it to float up and down freely with the tide.